Polymethylmethacrylate (PMMA) resins have long been used for the manufacture of contact lenses because of their excellent optical properties and machining and molding characteristics. A major disadvantage of PMMA resins is their very low permeability to gases such as oxygen. Since the cornea needs a continuous supply of oxygen from the air to provide for ongoing metabolic processes, the low gas permeability of the PMMA resins has necessitated lens designs which ameliorate this problem to some degree. Design changes have included reducing the diameter of the lenses in order to decrease the amount of corneal area covered by the impermeable material and shaping the back surface of the PMMA contact lens to provide for a pumping action and concomitant tear flow under the lens, the tears containing dissolved oxygen from the air.
While such designs have made possible the wearing of contact lenses, significant problems and limitations remain, both because of the inadequacy of the oxygen supply to the cornea and because the designs may produce discomfort and undesirable physiological symptoms to the wearer, frequently to a degree which makes wearing of the contact lens possible for only short periods of time or not at all.
Continued oxygen deprivation of the cornea results in edema or swelling of the cornea by excess water. This impairs vision and may result in corneal damage. In addition, while oxygen must be supplied to the cornea for its metabolic processes, carbon dioxide, a waste product of these processes must be removed. The same principles apply for providing a route for removal of carbon dioxide from the cornea as for the transport of oxygen to the cornea, when a contact lens covers the cornea.
The ideal material would provide oxygen transport to the cornea equivalent to that without a lens present on the cornea. It has been found, however, that the cornea can remain healthy with an oxygen delivery lower than this, provided the continuous lens wear time is appropriately curtailed. It has been well established, however, that the higher the gas permeability, the greater the safety margin for retaining a healthy cornea, the greater the patient tolerance for the lens and the longer the continuous wear time of the lens by the patient.
Recently, materials other than PMMA and with higher gas permeabilities have been used for contact lenses. These include soft hydrogels, soft hydrophobic polysiloxane resins, cellulose acetate butyrate (CAB) resin, and 4-methyl pentene-1 polymer. The soft materials are very flexible. CAB is less rigid than PMMA, and poly(4-methyl pentene-1) is more flexible than PMMA and can best be characterized as semi-rigid. All other factors being equal, the softer and more flexible the material, the greater its wearing comfort is likely to be, although beyond a certain softness there would be no advantage.
While these other recent materials provide a significant improvement in gas permeability, they still do not provide an adequate gas permeability except for the polysiloxane lenses or hydrophilic lenses with very high water content. Lenses made of polysiloxane resins are not successful, however, as a contact lens material because of their high hydrophobicity, making it difficult or impossible for the tears of the eye to wet the lens, thereby causing great discomfort to the wearer. Hydrophilic lenses of very high water content are very weak. An important limitation of contact lenses made from the flexible materials is that in forming closely to the shape of the cornea, they do not correct for corneal astigmatism or other corneal conditions for which the lenses are worn as do the lenses made from more rigid materials. Further, each of the other recent materials noted above present various problems in forming, machining, and polishing the contact lens, as well as in wearing, handling and hygienic care of the lens by the patient.